1. Field of the Invention
The present invention relates to a p-type zinc oxide (ZnO) based oxide composition, and a production method thereof.
2. Description of the Related Art
Illumination using a light emitting diode (LED) has recently been attracted attentions, because of awareness of environmental issues, and measures for achieving low energy consumption. In order to spread use of the LED even wider, it is necessary to reduce a cost, and provide stable supply. An inorganic LED typically uses GaN as a main material, but Ga is a rare metal, and is extremely expensive, i.e., 140 JPY or higher per 1 g (at the time of 2008). The acquisition of rare metals is getting more difficult because of the export restriction set by China, which is a producing country of rare metals. Therefore, a development of the LED using an inexpensive material, which can replace Ga, is an immediate need.
Meanwhile, ZnO is inexpensive. Similarly to GaN, the ZnO is a direct transition semiconductor. Therefore, the ZnO has been attracted attentions as a light emitting material for a long time. However, a development of the LED using the ZnO has not been made progress. One of the reasons is that it is difficult to produce a p-type ZnO. This is because a donor, which is an oxygen defect, tends to be preferentially generated in the ZnO, and therefore it is difficult to introduce an acceptor using a typical method.
Various single atom acceptors, such as nitrogen (N), lithium (Li), phosphorus (P), arsenic (As), and antimony (Sb), have been studied so far, but a definite acceptor has not yet been found. For example, nitrogen cannot increase a hole density as an acceptor level thereof is deep, i.e., 0.2 eV. Therefore, there is a problem that an electrical resistivity cannot be reduced (K. Tamura, T. Makino, A. Tsukazaki, M. Sumiya, S. Fuke, T. Furumochi, M. Lippmaa, C. H. Chia, Y. Segawa, H. Koinuma, M. Kawasaki, Solid State Communications 127, 265 (2003)). Moreover, Li can act as an acceptor with substituting the Zn site. However, an interstitial Li atom tends to be generated at the same time. Therefore, there is a problem that Li cannot function as an effective acceptor (C. H. Park, S. B. Zhang, and Su-Huai Wei, Phys. Rev. B 66, 073202 (2002)). Moreover, there is a report that a p-type ZnO has been realized by adding the group V elements other than nitrogen, e.g., P, As, and Sb, and a LED has been able to be produced experimentally (S. Chu, J. H. Lim, L. J. Mandalapu, Z. Yang, L. Li, and J. L. Liu, Appl. Phys. Lett. 92, 152103 (2008)). It can be understood that P, As, and Sb act as a donor with substituting Zn sites, and a complex with a Zn defect (VZn) is acting as an acceptor. However, it has been theoretically pointed out that generation of such a complex is hard to occur (Sukit Limpijumnong, S. B. Zhang, Su-Huai Wei, and C. H. Park, Phys. Rev, Lett. 92, 155504 (2004)), and an origin of the acceptor is unknown. Moreover, further experiments conducted by other research groups have not been succeeded, and therefore there is a problem in reproducibility.
It has been found out from various researched conducted so far that it is difficult to provide a single acceptor to a ZnO lattice through doping, and the acceptor level thereof is deep. In order to overcome these problems, simultaneous doping where a donor and an acceptor are added at the same time, cluster doping where a donor-acceptor complex is provided by doping, and delta doping where an acceptor is added periodically have also been studied. As a result of these new methods, a concentration of an acceptor can be made high, and the depth of the level can be made slightly shallow. However, the effects thereof are not significant.
As for a method for realizing a shallow acceptor level, other than a method for changing a type of a dopant, there is a method where the energy of the upper end of the valence band of ZnO, which is a host, is increased. ZnO1-xSx, which is a mixed crystal of ZnO and ZnS, reduces its band gap in the range of 0<x<0.5. However, it has been known that this reduction is caused as only the energy level of the valence band eased (C. Persson, Phys. Rev. Letts., 97, 146403 (2006)). From this, it is considered that formation of a shallow acceptor level can be realized by using ZnO1-xSx as a host, and providing an acceptor to ZnO1-xSx through doping. In fact, it has been theoretically pointed out in C. Persson, Phys. Rev. Letts., 97, 146403 (2006) that an acceptor level of nitrogen is made significantly shallow. However, it has not yet reported that a p-type ZnO1-xSx film is realized by nitrogen doping.